This invention relates to a manufacturing method of semiconductor nonvolatile memory device capable of rewriting data electrically, and more particularly to a method of forming a tunnel oxide film of a floating gate type (FLOTOX type) memory device.
Conventional nonvolatile memory devices may be roughly divided into two types. One is a device called MNOS type, in which a thin tunnel oxide film region is formed in a part of a gate oxide film, a silicon nitride film is deposited on the tunnel oxide film region, and a semiconductor film of polysilicon or the like is deposited on this silicon nitride film, and this semiconductor film is used as the gate electrode. A high voltage of about 20 V is applied to the gate electrode, and electrons are accumulated in the electron trap in the silicon nitride film, at the interface trap level between the silicon nitride film and the tunnel oxide film. The other type is a device called floating gate type (FLOTOX type), in which a thin oxide film region of about 10 nm is formed in a part of a gate oxide film, and a semiconductor film such as polysilicon completely coated with an insulating film is formed on the gate oxide film in a shape of an island. The semiconductor film that is formed like an island and is floating electrically is generally called a "floating gate." On this floating gate, an insulating film is formed, and an upper gate electrode made of polysilicon or the like is formed. A high voltage of about 20 V is applied to the upper gate electrode, and electrons are injected into the floating gate by tunneling through a thin oxide film region of about 10 nm in order to charge the floating gate electrically, thereby data is written. Therefore, generally, the thin oxide film region of about 10 nm is called a tunnel oxide film. Up to now, the floating gate type memory device is manufactured by the steps for forming a relatively thick oxide film (30 to 130 nm) as a gate oxide film, removing the gate oxide film in the region for forming the tunnel oxide film by etching, exposing the silicon substrate surface in the region for forming the tunnel oxide film by using a patterned photoresist as the mask, and forming the tunnel oxide film (2 to 12 nm) by oxidation process. FIG. 3 shows the manufacturing method of floating gate type semiconductor nonvolatile memory device by such conventional method.
First, as shown in FIG. 3 (2), a P-type silicon substrate 1 is prepared. The P-type silicon substrate 1 is of 30 to 50 ohm-cm in order to raise the memory reading speed. As shown in FIG. 3 (a), a gate oxide film 2 of 30 to 1300 nm is formed on the silicon substrate 1. Afterwards, on the surface of the gate oxide film 2, a photoresist 3 is deposited, and the photoresist 3 is patterned and only the photoresist in the region for forming the tunnel oxide film is removed.
Using the photoresist 3 as the mask, the gate oxide film 2 is partially removed generally by wet etching. Afterwards, leaving the photoresist 3, As (arsenic) or P (phosphohorus) ions are injected into the surface of the silicon substrate 1, and a region 4 of As or P ion injection is formed as shown in FIG. 3(b), which shows the sectional view after removing the photoresist 3. The gate oxide film 2 is not present on the ion injected region 4, and the surface of the silicon substrate 1 is exposed in this region 4.
Provided that the N-type ion injection for formimg the ion injected region 4 in FIG. 3 (b) may be replaced by the ion injection into an N-type impurity region 5 shown in FIG. 3 (c), depending on the shape of the floating gate 7a to be formed in a later process.
Next, in the second gate oxidation process shown in FIG. 3 (c), a tunnel oxide film 6 is formed on the surface of the ion injected region 4. Consequently, polysilicon is deposited on the entire surface of the gate oxide film 2 and the tunnel oxide film 6. Then the impurity is diffused on the polysilicon, and the polysilicon is patterned. As a result, the gate electrodes, namely the floating gate 7a, and the selection gate 7b for writing and reading data are formed.
After forming the floating gate 7a and the selection gate 7b by patterning the polysilicon, ions of P or As are injected in the silicon substrate 1 at a concentration of about 1.times.10.sup.14 ions/cm.sup.2 by using the patterned photoresist for patterning the floating gate 7a and the selection gate 7b as the mask. As a result, the N-type impurity regions 5 are formed in the silicon substrate 1 as shown in FIG. 3 (c). The N-type impurity regions 5 contribute to enhancement of the drain breakdown voltage of the selection gate 7b, and also function as the diffusion layer for drifting the electrons to the tunnel oxide film 6 immediately beneath the floating gate 7a. Therefore, the concentration of the N-type impurity regions 5 cannot be raised too much. Incidentally, as explained hereinabove, if the central N-type impurity region 5 in FIG. 3 (c) reached up to the ion injected region 4 in FIG. 3 (b) by heat diffusion, the ion injection process for forming the ion injected region 4 may be omitted. The oxide film thickness of the tunnel oxide film 6 grown in the second gate oxidation process must be in a range of 5 to 15 nm. Usually it is about 10 nm. The area of the tunnel oxide film 6 is required to be small in consideration of the characteristics of operation of the FLOTOX type nonvolatile memory. Therefore, usually it is about 1 .mu.m square which is the minimum size of exposure apparatus at present.
In sequency, as shown in FIG. 3 (d), the surface of the floating gate 7a and selection gate 7b is oxidized, and a second gate oxide film 8 is formed. Then, growing again polysilicon on the surface of the second gate oxide film 8, the polysilicon is patterned, and a second gate called control gate 9 is formed. Furthermore, as shown in FIG. 3 (e), the gate oxide film is removed from the unnecessary portion on the surface of the silicon substrate 1, and then, ions of As, P or the like are injected in order to form highly concentrated N-type diffusion layers 10. The highly concentrated N-type diffusion layers 10 function as source, drain of MOS transistor.
Finally, as shown in FIG. 3 (f), the entire surface of the silicon substrate 1 including the control gate 9 is coated with an insulating film 11. Usually, this insulating film 11 is formed by the oxidation treatment of polysilicons 7a, 7b, 9, and oxidation treatment of the surface of the silicon substrate 1.
After FIG. 3 (f), same as in the process of ordinary semiconductor device, a thick insulating film is further deposited on the surface of the silicon oxide film 11, and contact holes are opened in the insulating film. Then the highly concentrated N-type diffusion layers 10 and the polysilicon gates 7a, 7b are connected to the metal wirings formed on the insulating film through the contact holes, so that the nonvolatile memory is completed. In almost all cases, needless to say, a final protective film is deposited on the device surface.
In the prior art, the tunnel oxide film 6 is a very thin oxide film of 5 to 15 nm, and it is difficuly to obtain a stable and uniform film thickness by the ordinary oxidation method. Accordingly, the oxygen partial pressure of the oxidation atmosphere is diluted with inert gas such as Ar (argon), He (helium) and N.sub.2 (nitrogen), or the oxidation temperature is lowered to oxidize the gate, thereby forming an ultrathin tunnel oxide film 6. Besides, a film thickness of as much as 30 to 150 nm is required for the gate oxide film 2, other than the tunnel oxide film 6, beneath the floating gate 7a, and therefore in order to form such a thick gate oxide film 2 beneath the floating gate 72, two steps of oxidation, one step of photoresist patterning for defining the region of tunnel oxide film 6, and one step of etching of the first gate oxide film for using the photoresist pattern as the mask are required. Furthermore, when forming the tunnel oxide film 6, since the thick gate oxide film 2 is formed in other regions than the region for forming the tunnel oxide film 6, a stress is caused on the silicon substrate surface in the boundary portion between the tunnel oxide film 6 and the surrounding thick gate oxide film 2 at the time of growth of the tunnel oxide film 6, and therefore the oxide film quality of the tunnel oxide film 6 deteriorates. As a result, the number of times capable of rewriting data is decreased, and therefore, it is difficult to obtain a favorable data rewriting characteristics as a semiconductor nonvolatile memory device.